Gas Furnace Troubleshooting Tips

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Common Gas Furnace Troubleshooting

These easy to understand troubleshooting tips are for common issues that any homeowner can check before making an appointment for service or repairs and to help understand what may be wrong. Please do not perform these tips if you are not comfortable performing them. Electricity is dangerous and can cause serious injury or death, always assume electrical power is on unless you have checked it with an appropriate electrical meter. Ameri-Serv, Inc. assumes no liability for the use of the information provided on this website. Before beginning please read the following article. Understanding Electrical Circuits and Standard Household Voltages

Checking your Control Thermostat

    * Is the thermostat's system selector switch in the "heat" position?
    * Is the thermostat's temperature set point at least three degrees higher than the temperature inside?
    * Is your thermostat is electronic? Some electronic  thermostats operate from batteries and if they okay the LCD display should be visible. Many times if the LCD display is not visible this may also indicated that the power to the furnace is off (see below). If you think you have a thermostat that runs off of batteries you can usually change the batteries by carefully removing the thermostat from the wall mount as the batteries are generally located inside the thermostat.
    * You can test your thermostat if it has switch or button to control the indoor blower or fan, switch it to "on." Listen to hear if the blower or fan comes on. If you do not hear any fan or there is no air coming from your supply registers then check to be sure the power is on at the indoor unit. (see below) If the power is on and you do not hear the fan running this may indicate that you have a bad capacitor, blower motor or other common control issue.
    * Is your thermostat working have you checked to be sure the thermostat call for heat is present at the furnace?


Checking your Power or Electrical

   * Your Gas Furnace may have a power switch at the indoor unit. In most homes the power switch looks just like a regular light switch and is generally located right on or beside the indoor unit. Make sure the switch is on as indicated above.
    * Most Gas Furnaces have another hidden switch that shuts off your unit if the front service panel or blower door is removed. Make sure all the service panels are on and secure.
    * If the fan still does not operate check to see if any electrical panel circuit breakers are tripped. Usually if a circuit breaker has tripped it will not be fully in the "on" position. To reset the circuit breaker turn it off all the way and then back on.


Checking your Gas Furnace Ignition

   * If your Gas Furnace is an older gas furnace (usually 10 plus years old) it may have a standing pilot light like a water heater, if this is the case carefully follow the instructions on the furnace to light the pilot.
    * Some models have an intermittent pilot light or spark ignition and you can usually hear the the system try to light the pilot flame with a spark (makes several clicking sounds).
    * Most newer Gas Furnaces these days have a hot surface ignitor which will glow red hot when the system is trying to light the main burner flames. The hot surface ignitor looks like a very small electric oven heating element and they are fragile and easily crack and break over time.
    * On all but the oldest furnaces you will have what is called an inducer fan which runs prior to the flame being lit, this is small fan in the inside of the unit and if it does not start and run when the system is trying to start and heat then you more than likely have a bad inducer fan motor or other possible safety switch problems and you should call a qualified service company. 


Gas Furnace Typical Components

In all cases if your furnace fails to light the pilot, the pilot will not stay lit or the main burner flame does not ignite you should shut off your furnace and contact a qualified service company to inspect your system. If you smell gas open an outside door and/or windows and exit the area immediately.






The furnace on the left is typical for 80% gas furnaces that are about 10 to 15 years old. The locations of the specific components vary somewhat for different manufacturers but this provides a good idea of where to find specific items.